Field of the Invention
The invention relates to a method for acquiring a magnetic resonance signal from an examination subject according to a pulse sequence, in particular a voxel-selective spectroscopy sequence, in which a phase rotation is used. This means that the sequence is repeated a number of times with respectively different phases of the radio-frequency pulses and the signals acquired in the process are added together.
Description of the Prior Art
Phase rotation is generally known in magnetic resonance (MR) and in particular in magnetic resonance spectroscopy. In the simplest spectroscopy data acquisition consisting of 90° excitation pulse followed by acquisition of the FID (Free Induction Decay), the phase of the excitation pulse can be incremented by 90° for example in each acquisition; at the same time the receiver phase is also incremented by 90° in each case. Since in this way the phase difference between the receiver and the magnetization of the FID remains constant in each case, each step in the phase rotation cycle produces the same lineshape of the resonance, with the result that the wanted signal is amplified during the summation, whereas other influences such as phase shifts due to amplifiers and filters cancel one another out.
In voxel-selective MR spectroscopy (Single Voxel Spectroscopy (SVS)), phase rotation is furthermore used in order to suppress unwanted coherences and interfering echoes (“spurious echoes”). These are produced, for example, due to the fact that the respective slice-selective excitation pulses in SVS do not excite the spins within the selected slice exclusively, but also act, albeit at a very much lower flip angle, on the spins outside of the voxel. In actual fact, the echoes generated thereby from outside of the selected voxel have a very much lower signal strength than the desired echo from the selected voxel. However, because the selected voxel usually has a very much smaller volume (e.g. 1 to 8 cm3) than the surrounding tissue, which contributes toward the unwanted echoes (e.g. the whole of the remainder of the skull), the unwanted echoes and coherences in the acquired signal also still possess a high signal strength nonetheless.
A phase rotation scheme for the PRESS (Point Resolved Spectroscopy) sequence was presented by J. Hennig in the Journal of Magnetic Resonance, 96, 40-49 (1992). The Press sequence consists of a slice-selective 90° pulse, followed by two 180° pulses, which are in each case emitted while a gradient field is applied in the two other spatial directions and lead to a voxel-selective spin echo. Minor inaccuracies in the phase of the radio-frequency (RF) pulses or their amplitude, which can apply in particular also to the volume outside of the selected voxel, lead to the outcome that not just the wanted echo is generated by means of the pulse sequence, but e.g. a spin echo is generated for each pair of pulses, and an FID for each pulse. These unwanted signals can be suppressed in part by means of spoiler gradients.
The phase rotation used by J. Hennig proceeds according to the following scheme:
Excitation pulse90°90°90°90°270°270°270°270°1st 0° 90°180°270°0°90°180°270°refocusing2nd90°180°270° 0°0°90°180°270°refocusing
This 8-step phase rotation is also referred to as 8-step EXOR.
Particularly in the case of high field strengths, it is often difficult to provide refocusing pulses having a sufficiently high bandwidth. For this reason, adiabatic refocusing pulses are frequently employed in such a situation. These are also referred to as AFP (Adiabatic Full Passage). The rapid adiabatic passage leads to a reversal of the spin orientation, which is less sensitive with respect to the magnetic field homogeneity. An adiabatic pulse is able to invert large bandwidths at low power. It is possible to replace the refocusing pulses in a PRESS sequence by adiabatic pulses. However, since each individual adiabatic 180° pulse having a slice-selective gradient generates a first-order phase variation across the spectrum, two successive adiabatic refocusing pulses are used in each case for each spatial direction. In LASER and semi-LASER spectroscopy, therefore, four adiabatic refocusing pulses are used instead of the two standard refocusing pulses in a normal PRESS sequence.
A phase rotation scheme for a semi-LASER sequence is proposed in V. O. Boer et al., NMR in Biomedicine 24 (9): 138-46 (2011). Said sequence consists of a two-step phase cycle of the excitation pulse (0°-180°), a two-step phase cycle of the third refocusing pulse (0°-180°), and a four-step phase cycle of the last refocusing pulse (0°-90°-180°-270°). This scheme cannot be shortened or modified in any other respect, however.